It has long been known that the electrical resistivity at a given temperature of any of a number of metal oxides is a function of the partial pressure of oxygen in the gaseous atmosphere or environment to which the metal oxide is exposed. Among the metal oxides having this characteristic are the transition metal oxides such as titanium dioxide, vanadium oxide, chromium oxide, manganese oxide, cobalt oxide, nickel oxide, iron oxide, and rare earth oxides an example of which is cerium oxide. This characteristic of such oxides has led to the suggestion of using same as a sensor element to determine the composition of internal combustion engine exhaust gases. Such systems, as well known in the prior art, comprise the sensor element, formed of a body of the metal oxide, located in the exhaust gas stream and having connected thereto a pair of electrical leads suitable connected, in spaced relationshipto each other, to a relationship to oxide body for measuring the electrical resistance of the sensing metal oxide between the two leads. The information so gathered, concerning the composition of the exhaust gases and hence the proportion of unburned or only partially burned hydrocarbon therein, can be fed to a suitable circuit and control mechanism to control the air/fuel ratio of the mixture fed into the combustion chamber or chambers of the internal combustion engine.
It is also well recognized in the prior art that the sensing metal oxide should be heated to a relatively high temperature in order to exhibit the sensing characteristics. Further, since the electrical resistance of the metal oxide is also sensitive to changes in the temperature, it is recognized that the metal oxide should be maintained at constant temperature when functioning as a sensor for the gas composition as aforesaid. Hence, it is known to use a heater element adjacent the metal oxide sensor element and to use a thermocouple, also adjacent the sensor element in order to measure the temperature and feed this information to a circuit for controlling the heater element thereby to maintain the temperature constant.